1. Field of the Invention
The present invention relates to a reinforced ion exchange membrane and a process for producing the same. More particularly, the present invention is concerned with a reinforced ion exchange membrane comprising a fluorinated ion exchange resin and a woven reinforcing fabric, in which the difference between the thickness of the membrane at the crossover points of the warp and weft strands of the fabric and that at the window portions of the fabric which are defined by the crossed warp and weft strands, is in a limited range. The present membrane is advantageously reinforced for use in separating the anode and cathode compartments of an electrolytic cell, especially a chloralkali electrolytic cell, which not only contributes to a lowered electric power consumption but also enables the operating voltage to be lowered. The present invention is also concerned with a process for producing an advantageous membrane in which an assembly comprising at least two melt-fabricable fluorinated polymer films, a support sheet superimposed on the films and a woven reinforcing fabric sandwiched between the films is heated while applying a negative pressure to one side of the assembly remote from the support sheet.
2. Discussion of Related Art
It is well known in the art that a cation exchange membrane of a fluorinated polymer having carboxylate and/or sulfonate groups can be advantageously employed to separate the anode and cathode compartments of an electrolytic cell, especially a chloralkali electrolytic cell. Naturally, the cation exchange membrane for use in this field of application must exhibit low power consumption and must have high mechanical strength such that the membrane is not damaged during the installation into a cell and the electrolytic operation. However, generally, the tear strength of a fluorinated polymer film is low, so that the film per se cannot endure being used in an electrolytic operation for a prolonged period of time. Accordingly, a countermeasure has been taken, in which a reinforcing material such as a woven reinforcing fabric is encapsulated in a fluorinated polymer film to thereby improve the tear strength of the film. This countermeasure is however often encountered with various problems. One of the problems is the current shielding which is attributed to the ion impermeability that is inherent of the woven reinforcing fabric. Another problem is that the thickness of the membrane becomes large at the crossover points, the points where the warp and weft strands of the woven reinforcing fabric cross (hereinafter often simply referred to as "crossover points"), due to the encapsulation of the fabric, which causes regions of high electrical resistance to be formed in the membrane. A further problem is that the thickness of the membrane becomes small at the window portions of the membrane, which are defined by the crossed warp and weft strands of the woven reinforcing fabric, which causes concave portions to be formed on the surface of the membrane, in which concave portions gas bubbles are trapped in the membrane for separating the anode and cathode compartments of an electrolytic cell, during the use of the cell in an electrolytic operation, thereby causing the cell voltage to be disadvantageously increased.
To cope with these problems, a process has been proposed in EP-A-O 031 724. In the proposed process, at least two films of melt-fabricable fluorinated polymer having pendant sulfonyl and/or carboxyl groups in a melt-fabricable form and a woven reinforcing fabric are brought into face-to-face contact such that proximate surfaces of two of the films contact opposite planar surfaces of the fabric, and air is removed from between the films at the two opposite edge portions thereof, followed by heat application to the two outermost opposite planar film surfaces. The membrane thus produced is subjected to a conversion reaction to convert the sulfonyl and/or carboxyl groups to an ion exchange form. With respect to the membrane produced according to this method, each window portion has a relatively uniform thickness but the difference between the thickness of the membrane at the crossover points where the membrane has the maximum thickness and that at the window portions where the membrane has the minimum thickness is so disadvantageously large that a flat membrane surface cannot be obtained. This is attributed to the fall of the fluorinated polymer film into the window portions to produce the above-mentioned thickness difference results, which fall would occur due to the differential pressure during the step of removing air from between the films. Consequently, when the membrane produced according to EP-A-O 031 724 is used to separate the anode and cathode compartments of a chloralkali electrolytic cell, hydrogen gas bubbles are likely to be trapped in the concave portions of the membrane surface on the side of the cathode compartment, thereby causing the electric power consumption to disadvantageously increase.